Joint prosthesis having PTFE cushion

ABSTRACT

A joint prosthesis for interconnecting the ends of adjoining bones includes a cushion of expanded PTFE having at least two facets for attachment to the bones. The cushion may or may not include stems for insertion into the bones. An optional sheath surrounds the non-facet surfaces of the cushion for preventing adherence of tissue. The motion of the prosthesis is defined or limited by the orientation of the facets and/or by the porosity of the cushion and/or by an insert within the cushion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a file wrapper continuation of patent applicationSer. No. 08/608,538, filed on Feb. 28, 1996, now abandoned, which was acontinuation of patent application Ser. No. 08/306,725, filed on Sep.15, 1994, which issued as U.S. Pat. No. 5,507,823 on Apr. 16, 1996; inturn, patent application Ser. No. 08/306,725 was a file wrappercontinuation of patent application Ser. No. 08/173,979, filed on Dec.28, 1993, now abandoned.

In addition, applicants filed a divisional patent application Ser. No.08/304,935, filed Sep. 13, 1994, based upon the aforementioned patentapplication Ser. No. 08/173,979, filed on Dec. 28, 1993; such divisionalpatent application Ser. No. 08/304,935 later issued as U.S. Pat. No.5,491,882 on Feb. 20, 1996.

BACKGROUND OF THE INVENTION

This invention relates to reconstructed joints between bones and, inparticular, to an improved prosthetic joint having a mass of expandedPTFE as the articulating, connective material.

In current medical practice, a joint severely afflicted with disease orinjury is replaced with one of a variety of prostheses. One type ofprosthesis includes metal socket elements which are shaped similarly tothe adjoining ends of healthy bones and have stems for insertion intobones. This type of prosthesis is used for a joint subjected to heavyloads, such as the hip joint. Another type of prosthesis, typically usedfor replacing damaged joints between bones subjected to minimal loads,includes an elastomer hinge for coupling the bones. This type ofprosthesis is less expensive than the first type and is more suited forsmaller joints, e.g. the metacarpophalangeal (MCP) joints in the secondthrough fifth fingers.

It is desired to restore the integrity and functionality of a joint asmuch as possible. For joints normally having limited motion, this goalis relatively easily attained. For joints capable complicated motions,such as the bi-axial motion of the fingers, this goal becomes far moredifficult. The bi-axial motion of a thumb is readily demonstrated byplacing a hand flat upon a flat surface. Moving the thumb toward thepalm (adduction) or away from the palm (abduction) while the thumb restson the surface is motion about one axis and moving the thumb up from thesurface (extension) or into the surface (flexion) is motion about thesecond axis.

Movement about two axes can be achieved with mechanical prostheses butthese are often too expensive and of limited reliability. Prosthesesmade with elastomer hinges either do not provide bi-axial motion orpermit unnatural motion of the bones. Either type of prosthesis in theprior art provides a hinge or swinging kind of motion, not the slidingmotion of a natural joint.

Mechanical, i.e. metal, joints are often fastened to the bones with abone adhesive. However, the mechanical joints are not resilient and theshock transmitted to the joint often causes the adhesive layer to crackor to remove a thin, adherent layer of bone tissue. In addition, thebone adhesive deteriorates with time. As a result, the joint musteventually be repaired or replaced.

Elastomer hinges have the advantage of being resilient but are rapidlyfalling into disfavor because they are typically made from silicone,which has been found to be incompatible with the tissue surrounding animplant. Silicone can break down into small particles that are ingestedby body cells, causing an inflammation known as "silicone synovitis".The flexible web connecting the two halves of the hinge can fracturefrom repeated use and the whole prosthesis may have to be replaced.Also, silicone in prolonged contact with bone can cause the bone todissolve.

All known prostheses currently used have limited life in the patientbefore the problems described above begin to appear, sometimes as soonas one year after a joint is replaced. A problem that occurs immediatelyupon replacement of a joint is the body's healing process which fills ajoint with fluid. Drains to reduce swelling can help but therapy muststart as soon after surgery as possible to minimize formation of scartissue which can freeze a joint.

Polytetrafluoroethylene (PTFE) is known to be chemically stable andbio-compatible and expanded PTFE has been used in either tubular orsheet form for vascular grafts. In non-medical applications, expandedPTFE is used as a gasket material for example.

As disclosed in U.S. Pat. No. 4,187,390 (Gore), expanded PTFE istypically made by a cold extrusion process in which a paste of PTFE andlubricant is forced through a die. The extruded PTFE expands and is keptexpanded during "sintering," i.e. heating the PTFE almost to its meltingpoint, approximately 340° C., and then allowing the PTFE to cool. Aftersintering, the extrusion is cut into sections and is ready for use. Ifsqueezed between the fingers and released, expanded PTFE compresses andremains in its new shape until extended.

Expanded PTFE has a microscopic structure of nodes interconnected byfibrils and is porous. Porosity is measured as the average distance,e.g. 8-10 microns, from one node to another among a plurality of nodesmaking up a pore, i.e. porosity is a measure of fibril length. Theelastic deformation of millions of tiny fibrils permits the expandedPTFE to compress or stretch and return to its original shape.

In view of the foregoing, it is therefore an object of the invention toprovide a joint prosthesis having a longer life in the patient andimproved function.

Another object of the invention is to provide a joint prosthesis havinga more natural function.

A further object of the invention is to provide a simple jointprosthesis which can move in two axes.

Another object of the invention is to provide a joint prosthesis inwhich the cushion is preformed with stems for controlling the motion ofthe joint and enhancing the stability of the joint.

A further object of the invention is to provide a joint prosthesishaving high damping capability.

Another object of the invention is to provide a joint prosthesis that isbio-compatible with a patient.

A further object of the invention is to provide a joint prosthesishaving controlled adherence to surrounding tissue.

Another object of the invention is to simplify the installation of jointprostheses.

A further object of the invention is to provide a joint prosthesis whichcannot fill with body fluids.

Another object of the invention is to provide a prosthesis that is lessexpensive than prostheses of the prior art while providing the samefunction.

SUMMARY OF THE INVENTION

The foregoing objects are achieved by the invention in which a mass orcushion of expanded PTFE has at least two facets for attachment betweenthe prepared ends of adjoining bones. In accordance with one aspect ofthe invention, a portion of the cushion is molded into stems by heatingpart of the cushion and compressing the PTFE into a low expansion orunexpanded state. In accordance with another aspect of the invention,separate stems are attached to the cushion. In accordance with a thirdaspect of the invention, a stemless cushion has highly porous facets incontact with the prepared ends of the bones and the bone or fibroustissue adheres to the porous facets. Since the cushion is porous, anon-porous coating or a thin, non-porous sheath may be added to preventtissue adherence to the non-facet surfaces of the cushion.

The cushion permits motion along several axes and the orientation of thefacets relative to one another affects the motion of the prosthesis. Forexample, if the facets of the cushion are tilted, i.e. not parallel,then the prosthesis is particularly useful for the MCP joints of thehand which flex further than they hyper-extend. The prosthesis simulatesthe gliding motion of a natural joint by yielding to shear forces. Inaccordance with another aspect of the invention, a cushion includes aninsert to modify the motion of the prosthesis.

The joint can be installed using current techniques since the stems canbe molded in the same shape as stems currently used in other prostheses.Alternatively, a round hole is drilled into the bony canals and roundstems are inserted into the holes. Metal sleeves and grommets can beused to separate the PTFE from the bony canal. If a stemless cushion isused, the cushion is inserted between the treated ends of the bones anda cast immobilizes the joint until the bones have had a chance to adhereto the facets of the cushion.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention can be obtained byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIGS. 1 and 2 illustrate elastomeric hinges of the prior art;

FIGS. 3A and 3B illustrate the movement of a prosthesis having anelastomeric hinge;

FIGS. 4A, 4B,and 4C illustrate the motion of a natural joint;

FIG. 5 illustrates the motion of a MCP joint about two axes;

FIGS. 6 and 7 illustrates a prosthetic joint constructed in accordancewith the invention;

FIG. 8 illustrates a method for making a prosthetic joint in accordancewith the invention;

FIG. 9 illustrate the response of the cushion to shear;

FIG. 10 illustrates a prosthesis constructed in accordance with apreferred embodiment of the invention;

FIG. 11 illustrates the replacement of an MCP joint in a hand;

FIGS. 12A and 12B illustrate the motion of a prosthetic joint;

FIG. 13 illustrates a prosthetic joint constructed in accordance with analternative embodiment of the invention; and

FIG. 14 illustrates a stemless prosthesis constructed in accordance withthe invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a commercially available, elastomeric, hinge-typeprosthesis which includes plates 11 and 12 interconnected by web 13.Stem 14 is attached to plate 11 and stem 15 is attached to plate 12. Thestems are inserted directly into the bony canal or are inserted with ametal grommet or metal sleeve to protect the stems from wear by the boneduring movement. The stems are non-circular to prevent rotation ofprosthesis 10. Up and down motion, as prosthesis 10 is positioned inFIG. 1, is asymmetrical because the web is not centrally located. Web 13prevents side to side motion.

FIG. 2 illustrates a commercially available prosthesis which includesU-shaped elastomeric member 21 having stem 23 attached to one side andstem 25 attached to the other side of member 21. Although the axis ofrotation, indicated by reference numeral 26, is displaced from stems 23and 25, the motion of a joint using prosthesis 20 is nevertheless ahinge-type motion wherein stem 25 rotates about axis 26 relative to stem23. Prosthesis 20 is capable of considerable rotation as the U isopened.

FIG. 3A illustrates prosthesis 31 interconnecting bones 32 and 33. Thecenter of rotation of prosthesis 31 is indicated by reference numeral35. As shown in FIG. 3B, bone 32 pivots about axis 35 relative to bone33 moving the ends of the U toward each other at 36. There is no slidingmotion, as occurs in natural joints, and the center of rotation is notwithin either of the bones.

FIG. 4 illustrates the motion of two bones in a natural joint betweenbone 41 and bone 42. The center of rotation, indicated by referencenumeral 44, is within bone 41 away from the end of the bone. Thisreduces stress on the bones as they move. In FIG. 4B, bone 42 hasrotated about axis 44, sliding along the surface contact between the twobones.

In FIG. 4C, arthritis or other disease can cause the axis of rotation tomove from location 44 to a point between the bones indicated byreference numeral 45. When bone 42 can no longer slide across the end ofbone 41, bone 42 pivots about point 45 where the forces on the joint areconcentrated. This produces great stress on the bone tissue and isextremely painful for a patient.

The figures thus far have illustrated the simple motion of a joint abouta single axis. FIG. 5 illustrates the motion of an index finger rotatingthe MCP joint. The tip of index finger 48 traces out an ellipse havingX-axis 51 as the minor (shorter) and Y-axis 52 as the major (longer)axis. It is highly desirable to retain this motion for a patient uponreplacement of the MCP joint of the index finger.

FIGS. 6 and 7 illustrate a joint constructed in accordance with theinvention. Prosthesis 60 includes cushion 61 having stems 62 and 63extending from opposed facets thereof. Stems 62 and 63 have anon-circular cross-section to accommodate current practice forreconstruction of joints. It is not required by the invention that thestems have any particular cross-section.

Cushion 61 is a mass of expanded PTFE having the approximate consistencyof a marshmallow, i.e. it is a highly damped material with little"memory" of a previous shape. Highly damped means that a ball ofexpanded PTFE would not bounce well, if at all. Cushion 61 can becompressed somewhat and provides progressively greater resistance as itis compressed. It will also stretch slightly if one pulls on the joint.Since cushion 61 is porous, there may be a tendency for the surroundingtissue to adhere to the pores. In order to prevent adherence, cushion 61may include thin sheath 66 (FIG. 7) of non-porous or low porosity PTFEcovering all but the facets. A suitable material for the cushion isGoreTex® Joint Sealant as sold by Gore Associates as a gasket materialfor sealing pipes. A suitable sleeve is similar to vascular graftmaterial, also sold by Gore Associates.

Stems 62 and 63 can be formed as separate elements or, preferably, areintegral with cushion 61 and are formed by molding the ends of cushion61. FIG. 8 illustrates a molding operation in which a cylindricalsection of expanded PTFE has the ends thereof inserted into andcompressed by heated die 81. Heated die 81 raises the temperature of theexpanded PTFE approximately to its melting point, collapsing thematerial and simultaneously forming stem 62 and facet 68 and stem 63 andfacet 69. Facets 68 and 69 are stiff and transmit the forces applied tostems 62 and 63 over the entire cross-section of cushion 61. Asindicated in FIG. 7, central region 71 has a lower density (higherporosity) than end regions 72 and 73 and stems 62 and 63.

The motion of prosthesis 60 (FIG. 6) is not a simple rotation about anaxis through cushion 61. FIG. 9 illustrates the response of cushion 61to shear forces. A shear force is a sideward force on an object, such aspushing a stack of coins from one side. As illustrated in FIG. 9, facets68 and 69 are in parallel planes and cushion 61 is subjected to a shearforce, represented by arrow 93. The facets can move past each otherwhile remaining in their parallel planes. While force 93 is illustratedas acting in the plane of the drawing, the joint can respond to a forcein a direction perpendicular to the plane of the drawing, or anydirection in between. Since the motion in response to shear is combinedwith a tilting motion, the prosthesis reproduces very closely the motionof a natural MCP joint. As illustrated in FIG. 9, stems 75 and 76 areround.

In FIGS. 6-9, the opposed facets of cushion 61 are illustrated asparallel. For some applications, the facets are preferably tilted withrespect to each other, i.e. the facets lie in intersecting planes. FIG.10 illustrates a preferred embodiment of the invention which isparticularly suited to MCP joints which flex more than they hyperextend.As indicated by arrow 101, the upper or outer portion of cushion 102 iswider than the lower or inner portion, as indicated by arrow 103. Thetilt between the facets permits bone 104 to move upwardly about cushion102 relative to bone 105 less than it can move downwardly, as cushion102 is oriented in FIG. 10. The tilt thus enhances the ability of thereconstructed joint to flex and controls hyper-extension of thereconstructed joint.

Stem 106 is inserted into metal sleeve 107 in bone 105 and stem 108 isinserted into metal sleeve 109 in bone 104. The stems and sleeves have acomplementary, non-circular shape to prevent rotation of the cushion.

FIG. 11 illustrates the installation of a prosthesis at the MCP joint ofthe index finger of a left hand. The diameter of cushion 110, indicatedby arrows 111, is determined by the size of a patient's hand, as is thelength, indicated by arrows 112. Since the natural joint has a motion ofapproximately +50°, -90°, the facets of the cushion are not parallel toapproximate the same range of movement.

The motion of the prosthesis is illustrated in FIGS. 12A and 12B, whichshow a combination of shear motion and tilting motion in the cushion.During flexion, outer surface 115 is stretched while inner surface 116compresses, forming a plurality of folds. Extensor tendon 117 stretchesover the prosthesis in a natural curve. Outer surface 115 curvesoutwardly, giving extensor tendon 117 a better moment arm thanobtainable from existing prostheses, making it easier to return thefinger to a straight position. In addition, bone 104 moves around bone105 as if on a sliding joint.

With bones 104 and 105 approximately in a straight line, cushion 110changes shape as shown in FIG. 12B. Upper surface 115 becomes slightlyfolded and lower surface 116 becomes smooth. Further extension of bone104 becomes progressively more difficult because of the greaterthickness across the top of cushion 110. Thus, a cushion withnon-parallel facets simulates the motion of a natural joint.

The motion of a prosthesis constructed in accordance with the inventioncan be defined or limited by using non-parallel facets and/or by varyingthe porosity of the cushion. The motion of the prosthesis can also becontrolled by including an insert within the cushion. In FIG. 13 endplates 121 and 122 are connected by flexible web 123. Web 123 has arectangular cross-section and can bend more freely in an up and downdirection than in a direction approximately perpendicular to the planeof the drawing. Cushion 127 surrounds web 123 and provides theresiliency and damping as described in previous embodiments. Web 123preferably is made from a thin sheet of non-expanded PTFE. In apreferred embodiment of the invention, plates 121 and 122, web 123, andstems 128 and 129 are molded as a single piece from unexpanded PTFE.Expanded PTFE is then extruded about web 123 between plates 121 and 122,forming cushion 127. Alternatively, cushion 127 is slit and insertedabout web 123. A non-porous sleeve (not shown) may then be slid overcushion 127, completing the prosthesis. This prosthesis is more suitedto the proximal interphlangeal joints of a finger.

FIG. 14 illustrates an alternative embodiment of the invention whereinno stems are used and the treated ends of the bones are permitted toadhere to the facets of the cushion. Region 132 adjacent bone 133 andregion 134, adjacent bone 135, are more porous than central portion 137of cushion 131. Regions 132 and 134 are more porous to encourageadherence of bone or fibrous tissue, as indicated by arrows 138. Sheath139 surrounds the outer surface of cushion 131, thereby preventingadherence except at the facets. When this joint is used, the patient'sfinger is immobilized for a few weeks to permit the adherence of bone orfibrous tissue, just as a broken bone is treated. This joint is moresuited to the carpometacarpal (basilar) joint of the thumb.

A joint prosthesis constructed in accordance with the invention isbio-compatible with the patient and should have a long service life. Inaddition, the prosthesis provides a more natural function thanprostheses of the prior art. The inter-nodal distance determines thehardness of the expanded PTFE. Shorter inter-nodal distances produce arelatively hard, low porosity material while larger inter-nodaldistances produce a softer, high porosity, high damping material. It hasbeen found that this damping capability is particularly useful in jointssince it simulates the natural damping of cartilage between bones.

Having thus described the invention, it will be apparent to those ofskill in the art that various modifications can be made within the scopeof the invention. For example, the cushion can be any size appropriatefor a joint. The porosity of a stemless cushion can be uniformthroughout if a highly porous, very soft interface is needed. An insertcan be used in any embodiment of the invention. The porosity at whichadherence is promoted or prevented may vary with the age of the patient.A porosity greater than six microns is believed to permit adherence ofbone or fibrous tissue in adults while a porosity less than six micronsis likely to prevent adherence of bone or fibrous tissue.

While described in conjunction with prostheses for a human being, ajoint constructed in accordance with the invention can be used onanimals or on anatomical models to produce a life-like motion. A jointconstructed in accordance with the invention can also be used for jointsin robotic "hands" or end effectors where the joints can provide ahighly damped, less rigid touch for a robot. Materials other thanexpanded PTFE can be used provided that they have the same biologicaland physical properties as described above in connection with expandedPTFE.

What is claimed as the invention is:
 1. A joint prosthesis forinterconnecting the adjoining ends of metacarpophalangeal bones, saidprosthesis comprising:a cushion of expanded, high-dampingpolytetrafluoroethylene having a first facet and a second facet, saidcushion permitting damped motion between said first facet and saidsecond facet; a first stem integral with and extending from said firstfacet for insertion into a hole in the end of a firstmetacarpophalangeal bone; a second stem integral with and extending fromsaid second facet for insertion into a hole in the end of a secondmetacarpophalangeal bone; wherein said cushion serves to damp motionbetween said facets, thereby simulating a natural joint between saidadjoining ends.
 2. The prosthesis as set forth in claim 1 wherein saidfirst facet is parallel to said second facet.
 3. The prosthesis as setforth in claim 1 wherein said first facet is tilted with respect to saidsecond facet.
 4. A joint prosthesis for replacing a natural jointbetween a first bone and a second bone which can move relative to thefirst bone, said prosthesis imitating the motion of the natural joint byinterconnecting the adjoining ends of said first bone and said secondbone, said prosthesis comprising:a cushion of expanded, high-dampingpolytetrafluoroethylene having a first facet and a second facet, saidcushion permitting damped motion between said first facet and saidsecond facet; a first stem integral with and extending from said firstfacet for attachment to said first bone; and a second stem integral withand extending from said second facet for attachment to said second bone;wherein said cushion serves to damp motion between said facets, therebysimulating a natural joint between said adjoining ends.
 5. Theprosthesis as set forth in claim 4 wherein said first stem and saidsecond stem each have a circular cross-section.